FEATURES ... The Run 11 ofthe Tevatron Jean-Fran~ois Grivaz Laboratoire de I'Accelerateur Lineaire, Orsay igh energy physics is the denomination commonly used to H designate the physics ofelementary particles, the branch of physics which deals with the building blocks ofmatter (quarks and leptons, as it presently seems) and their interactions. The reason for this denomination is that high energy beams are nec­ essary to probe short distances, which is why higher and higher energyparticle accelerators have been and continue to bebuilt. As oftoday, the particle accelerator providingthe highest energies is the Tevatron, a proton-antiproton collider located at Fermilab [1] (the Fermi National Accelerator Laboratory) near Chicago, Illinois (Fig. 1). Bunches ofprotons and antiprotons circulate in opposite directions in a 6.28 km long ring, and collide head on at specific locations where two large and complex detectors, named CDF [2] and DO [3], register the outcome oftheir interactions. The Tevatron collider operated from 1992 to 1996, at a centre-of­ I A Fig. 1: An aerial view of the Fermilab site.The main mass energyl) (twice the beam energy) of 1.8 TeV, delivering to I components ofthe accelerator system are highlighted: in yellow, each experimentanintegratedluminosity(2) of120 pb-I • Themain , the Tevatron, a 2 TeV proton-antiproton collider; in red the initial achievement ofthis period, known as Run I, was the discovery of I injection system; in blue the newly constructed main injector and the long sought top quark [4], with a mass of174 Gev, in 1995. i recycler. The locations of the (OF and DO detectors are also With the discovery ofthe top quark, the only missing piece in I indicated. I the highly successful Standard Model of electroweak interac­ L tions(3) is the Higgs boson, a very peculiar object since all other particles, which would otherwise be massless, acquire their mass through their interactionwith it. Its discovery would therefore be ofparamount importance. Essentially all ofthe current experi­ mentalknowledge onthe Higgs bosonwas obtained at CERN, the - LEP1 • SLD Data laboratory ofthe European Organisation for Nuclear Research .... LEP2, pp Data near Geneva, where the LEP electron-positron collider was in operation from 1989 to 2000 at centre-of-mass energies up to 209 80.5 68% CL GeV. Until 1995, data on the Z boson, the neutral mediator ofthe weak force, were accumulated, allowing >'Q) precision tests ofthe Standard Model to (!) be performed. Consistencywithin this ....... 80.4 Th e suecess0 r framework ofthe results thus obtained(4) ~ required a top quark mass in the range E of LEP at CERN 155 to 185 Gev, preciselywhere it ended up being found at the Tevatron. Togeth­ 80.3 er with the measurements of the mass will be the LHC of the charged weak boson W per­ formed both at LEP and at the Tevatron, Preliminary 80.2 --j-,......:....,.:--""--r-;:<-..,-"-T'-=~~-.--.,.-.~~ (Large Hadron and of the top quark mass at the Teva­ tron, the same consistency requirement 130 190 210 Collider). ofthe StandardModel now predicts that the mass ofthe Higgs boson should be lower than 190 GeV [5] (Fig. 2). Finally, direct searches for the Higgs boson [6] ... Fig.2: Masses of the Wboson and ofthe top quark as performedat thehighest LEP energies exclude itfor masses below measured directly at LEP and at the Tevatron (in green), and as 114 GeV, however with tantalising hints around 116 GeV. indirectly predicted, based on precision measurements at the Z The successor ofLEP at CERN will be the LHC (Large Hadron resonance {in red); the contours drawn correspond to a confidence Collider), a 14 TeV proton-proton collider which was essentially revel of 68%. The yellow band represents the Standard Model designed inview ofthe discovery ofthe Higgs bosonorofnewpar­ prediction f~:lr various Higgs boson masses. Direct and indirect ticles predictedbytheories beyond the StandardModel, for masses measurements are in agreement, and point toward a light Higgs all the way up to 1TeV. The construction and commissioningofthe boson. (From Ref. 5.) LHC and ofthe associated detectors will however not be complet- europhysics news JANUARY/FEBRUARY 2003 9 Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn:2003102 FEATURES ed before 2007, at the earliest. Until then the Tevatron, which resumedoperationinthespringof2001, will remainwithoutrival at the high energyfrontier. It was furthermore realised in the late nineties [7] that a Higgs boson with mass in the range indicated byLEP direct and indirect searches, i.e. between 114 and 190 GeV; couldwell bewithin the reach ofthe Tevatron, provided thata suf­ ficient integratedluminosityis delivered (Pig. 3). Both the Permilab accelerator complex and the CDP and DO detectors were substantiallyupgraded for this newphase ofoper­ ation called Run IT. On the accelerator side,two entirely newrings, the MainInjector and the Recycler, were con- Bot h the Fer miIab structed, with the goal of increasing the antiproton intensity which had been accelerator complex the main luminositylimita­ tion during Run I. Together and the (OF and DO with other challenging beam optics improvements, d et e ctors we re these upgrades shouldallow I the instantaneous luminosi­ substantially upgraded I ~ Fig. 4: Aphotograph of the CDF detector, opened during the ty to be progressively j installation of the silicon detector. From the inside to the outSide, increased by a factor offive for this new phase ' the tracking chamber, the calorimeter and the muon chambers in a first phase, possibly of can be seen. up to fifteen in a second one. This would lead to an l integrated luminosity of 2 quark. The DO collaborationhad decided to rather emphasisepre­ tb-I (i.e., 2000 pb-I) collected by the end of2005 (Run IIa), and cise, homogeneous and hermetic calorimetry (Pig. 5). For Run IT, then ofup to 3 tb-I everyyear thereafter (Run lIb). In addition to the CDF central tracker was entirely rebuilt, and both the these luminosity improvements, the proton-antiproton collision calorimetryandmuon detection coverage extended.Inthe case of energyhas been increased to 1.96 TeY. DO, the muon coverage was also improved, but the main modifi­ The CDP and DO detectors had been initially optimised in cation has been the introduction ofa superconducting solenoid quite different ways. In the case ofCDP (Pig. 4), the emphasis had inside the calorimeter, andwithin it ofa totally new tracking sys­ been put on an excellent detection and measurement ofcharged tem. In particular, silicon detectors were also installed in DO, so particles in a large volume tracking chamber. Silicon detectors that the intrinsic capabilities of the two experiments are now placed near the beam pipe allowed secondary vertices to be much more similar than theywere during Run I. With the antici­ reconstructed. Such vertices may arise in particular from the pated luminosity, and therefore interaction rate, increases, the decay ofshort-lived particles such as those containing a bottom silicon detectors ofboth CDP and DO will have to be replaced at ---, the end of2005 by new"radiation hard" ones. i The Run 11 ofthe Tevatron will offer a broad range ofphysics opportunities: • The higherluminosityandthe increased production cross sec­ tion will allow a large sample oftop quarks to be collected. Because ofits large mass (forty times the mass of the second heaviest quark) , the top quark properties are quite unusual, -------------------- 2 fb-1 and their detailed investigation will become possible. In par­ - 95% CL ~mit ticular, the uncertainty on its mass could be reduced to - 30 evidence ±2GeY. - 5.:; discovery 80 100 120 140 160 180 200 • It should also be possible to reduce to ± 30 MeV the uncer­ 2 Higgs moss (GeV/ c ) taintyon the mass oftheW boson.As canbeinferred from Fig. 2, this improved precision ofthe top quarkandW bosonmass­ es would allow the mass ofthe Standard Model Higgs boson ... Fig. 3; Reach ofthe Standard Model Higgs boson seaKh atthe to be much better constrained. It could even well be that these Tevatron.The curves represent, as a function ofthe Higgs boson indirect constraints conflictwith the masslowerlimitobtained mass, the luminosity which has to be delivered to each ofthe two at LEP, which would point to some new physics beyond the experiments in order to allow an exclusion at 95% confidence level Standard Model. (in purple), a 3aevidence (in green), or a Sa observation (in blue) I to be achieved. (From Ref. 7.) • Direct searches for phenomena beyond the Standard Model '-- -.1i will also be pursued, greatly benefiting not only ofthe higher 10 europhysics news JANUARY/FEBRUARY 2003 FEATURES NORTH SOUTH :.~ . ~",- .\ m)o .... Fig. 5: Aschematic cut view of the DO detector,showing the various components. Inside out starting from -5 the beam pipe: the silicon and scintillating fiber tracking systems, the superconducting solenoid, the uranium­ liquid argon calorimeter, the iron toroid with the muon detectors. -10 -5 10 luminositybut also ofthe higher energy, compared to Run I. For reasons which cannotbe addressed in this shortarticle, it is Footnotes: commonly expected that the Standard Model, even if it is (1) Energies and masses are measured in electron-Volt (eV) units. One experimentally very successful, will break down at a scale not MeVisworth 106 eY,oneGeV 10g eY,andoneTeV 1012 eY. much beyond the few hundred GeV presently probed, there­ (2) Instantaneous interaction rates are proportional to the luminosity of 2 l fore possibly within reach ofthe upgraded Tevatron. the collider, measured in cm- s- , and to the cross section ofthe process 2 considered, measured in cm , or rather in pico- or femtobarns (10-12 or •A huge number ofbottom quarks will be produced, which will 10-15 barns, where one barn is worth 10-24 cm2). For an integratedlwni­ allow very difficult, but unique, measurements to be per­ nosityofN pb-1 (N fu-I),N events are produced bya process with a cross formed.